C. botulinum toxin complex
Toxins of the different C. botulinum serotypes are produced in culture as aggregates of neurotoxin and other non-toxic proteins non-covalently associated into a polypeptide complex (Schantz, E., Purification and characterization of C. botulinum toxins, In K. Lewis and K. Cassel, Jr. (eds.), Botulism. Proceedings of a symposium. U.S. Department of Health, Education, and Welfare, Public Health Service, Cincinnati, pp. 91-104, 1964; Sugii, S. and Sakaguchi, G., Infect. Immun. 12:1262-1270, 1975; Kozaki, S., et al., Jpn. J. Med. Sci. Biol. 28:70-72, 1974; Miyazaki, S., et al., Infect. Immun. 17:395-401, 1977; Kitamura, M., et al., J. Bacteriol. 98:1173-1178, 1969; Ohishi and Sakaguchi, Appl. Environ. Microbiol. 28:923-928, 1974; Yang, K. and Sugiyama, H., Appl. Microbiol. 29:598-603, 1975; Nukina, M., et al., Zbl. Bakt. Hyg. 268:220, 1987). Toxin complexes are described as M for medium, L for large and LL for very large. These toxin complexes vary in size from ca. 900 kD for type A LL toxin complex to ca. 300 kD for the type B M complex and type E complex, to 235 kD for type F M complex (Ohishi, I. and Sakaguchi, G., supra, 1974; Kozaki, S., et al., supra, 1974; Kitamura, M., et al., supra, 1969). According to Sugii and Sakaguchi (J. Food Safety 1:53-65, 1977), during culture the proportion of one toxin complex versus another is dependent on the growth medium and conditions. A type B culture grown in the presence of 1 mM Fe.sup.+2 produces an equal proportion of L and M complexes while the same culture grown in the presence of 10 mM Fe.sup.+2 produces predominantly M complex.
TABLE 1 ______________________________________ Molecular sizes of various C. botulinum toxin complexes. Sedimentation Toxin type coefficient ca. M.sub.r (kDa) ______________________________________ LL A 19S 900 L A, B, D, G 16S 450-500 M A, B, C.sub.1, D, 10-12S 235-350 E, F, G ______________________________________
Some of the non-toxic proteins associated with the various toxin complexes have hemagglutinating abilities (Sugiyama, H., Microbiol. Rev. 44:419-448, 1980; Somers, E. and DasGupta, B., J. Protein Chem. 10:415-425, 1991). In particular, non-neurotoxic fractions of the L complexes of type A, B, C, and D have been shown to have hemagglutinating activity. Hemagglutinin fractions isolated from the different serotypes show some serological cross-reactivity. Non-toxic fractions from type A and B serotypes cross-react (Goodnough, M. and Johnson, E., Appl. Environ. Microbiol. 59:2339-2342, 1993) as do non-toxic fractions from types E and F. The non-toxic fractions of types C.sub.1 and D are antigenically identical as determined by Ouchterlony diffusion (Sakaguchi, G., et al., Jpn. J. Med. Sci. Biol. 27:161-170, 1974).
The non-toxic complexing proteins have been demonstrated to be essential for stabilization of the toxin during passage through the digestive tract (Ohishi and Sakaguchi, supra, 1974; Sakaguchi, G., et al., Purification and oral toxicities of Clostridium botulinum progenitor toxins, In Biomedical aspects of botulism, G. Lewis (ed.), Academic Press, Inc., New York, pp. 21-34, 1981). Pure neurotoxin has a peroral LD.sub.50 about 100-10,000 times lower than that of toxin complex on a weight basis (Ohishi, I., Infect. Immun. 43:487-490, 1984; Sakaguchi, G., Pharmacol. Therap. 19:165-194, 1983). Presumably, the complexing proteins protect the very labile toxin molecule from proteolytic cleavage and other types of inactivation by enzymes present in the gut and circulatory systems since the toxin and the complexing proteins are very stable in low pH environments.
Analysis by SDS-PAGE has shown that type A toxin complex consists of seven different nontoxic proteins ranging in size from ca. 17 kD to 118 kD in association with a neurotoxic protein of ca. 147 kD (Goodnough, M. and Johnson, E., supra, 1993; Gimenez, J. and DasGupta, B., J. Protein Chem. 12:349-361, 1993; DasGupta, Canad. J. Microbiol. 26:992-997, 1980). Isolated type A toxin complex has a specific toxicity of 2-3.times.10.sup.7 intraperitoneal LD.sub.50 /mg in 18-22 g white mice. Specific toxicities of other C. botulinum toxin complexes are type B M complex- 4-5.times.10.sup.7 LD.sub.50 /mg, type C.sub.1 M complex- 1-2.times.10.sup.7 LD.sub.50 /mg, type D M complex- 7-8.times.10.sup.7 LD.sub.50 /mg, type E M complex- 1.times.10.sup.7 LD.sub.50 /mg, type F M complex- 2-3.times.10.sup.7 LD.sub.50 /mg (Sugiyama, H., supra, 1980), and 8-9.times.10.sup.6 /mg for type G complex (Schiavo, G., et al., J. Biol. Chem. 269:20213-20216, 1994).
C. botulinum neurotoxin.
The biologically active neurotoxin of C. botulinum is a dichain molecule of ca. 150 kD in molecular weight. The molecule is composed of two fragments or chains that are termed the heavy chain (Hc, ca. 100 kD) and the light chain (Lc, ca. 50 kD) that are covalently connected by one disulfide bond (FIG. 1). The neurotoxin is synthesized by the organism as a single polypeptide called the protoxin and undergoes posttranslational processing termed nicking to generate the two separate chains by at least one protease (Yokosawa, N., et al., J. Gen. Microbiol. 132:1981-1988, 1986; Krysinski, E. and Sugiyama, H., Appl. Environ. Microbiol. 41:675-678, 1981). The two chains are covalently bound through a disulfide bridge. The nicking event occurs in the culture fluid for proteolytic C. botulinum and through the activity of an exogenous enzyme such as trypsin in non-proteolytic strains (Yokosawa, N., et al., supra, 1986; DasGupta, B., J. Physiol. (Paris) 84:220-228, 1990; Kozaki, S., et al., FEMS Microbiol. Lett. 27:149-154, 1985).
Functional Domains of Botulinal Neurotoxin.
Binding to cell surface. The carboxyl terminus of botulinal heavy chain is responsible for receptor binding on the cell surface. Initial work done using tetanus toxin, which is very similar in structure to botulinum neurotoxin, showed binding to cell receptors involved a multiple step binding sequence. The ten C-terminal amino acids are essential for initial receptor recognition on the motor neuron via a low affinity binding site while a sequence in the middle of the heavy chain was responsible for higher affinity secondary binding through a different protein receptor (Halpern, J. and Loftus, A., J. Biol. Chem. 268:11188-11192, 1993).
Evidence shows that binding by type B botulinum neurotoxin occurs in a similar fashion (Nishiki, T., et al., J. Biol. Chem. 269:10498-10503, 1994). The binding of type B neurotoxin to synaptosomes has been shown to be related to the presence of sialic acid containing motor neuron membrane components such as gangliosides G.sub.DIa and G.sub.Tlb as well as a partially purified 58 kD protein that has been tentatively determined to be synaptogamin. There is minimal binding of the neurotoxin to the 58 kD high affinity receptor in the absence of the low affinity gangliosides. This indicates that the initial low affinity binding to gangliosides which are prevalent on the cell surface by the carboxyl-terminal amino acids is followed by a high affinity binding to the 58 kD protein by an undetermined region that is more amino terminal possibly in the central portion of the heavy chain. Treatment of synaptosomes with proteases and or sialidase decreased binding of the neurotoxin to the synaptosomes.
Channel formation. Once the neurotoxin is bound to the motor neuron via the C-terminus end of the heavy chain, the light chain and the N-terminus of the heavy chain are endocytosed. The proteolytically active light chain is then released into the cytosol of the cell via a translocation event through the phospholipid vesicle membrane. This translocation event is driven by a sequence of amino acids contained in the N-terminal portion of the heavy chain. The predicted sequence responsible for translocation of botulinum toxin type A is from amino acids 650-681 and shows strong sequence homology to tetanus toxin amino acids 659-690 (Montal, M., et al., FEBS. Lett. 313:12-18, 1992). Both of these regions contain a high number of hydrophobic amino acid residues which presumably facilitate intercalation into lipid bilayers.
Under the acidic conditions of the vesicle, channels form in the lipid bilayer due to the N-terminal portion of the heavy chain associating into a bundle of amphipathic alpha-helices. These bundles contain four heavy chain portions that allow the light chain to enter the cytosol as evidenced by conformational energy calculations and direct visualization (Montal, M., et al., supra, 1992; Schmid, M., et al., Nature 364:827-830, 1993). There are believed to be two different conformations of the channel which may begin forming soon after binding of the C-terminal portion of the heavy chain. One conformation is a low conducting version while the second has a much greater conductance in electrochemical studies (Donovan, J. and Middlebrook, J., Biochem. 25:2872-2876, 1986). The difference in the two conformations can be explained by the fact that there is a change in pH from the physiologic condition under which the toxin initially binds and conductance is low to the lower pH values of the endocytotic vesicle where conductance is higher. The rate of conductance through channels has been shown to be highest at a pH of about 6.1 and lower at pH values closer to neutral (Donovan, J. and Middlebrook, J., supra, 1986).
Enzymatic activity in neuron/specificity for substrate. In order to describe the mechanism of botulinum neurotoxins in general, the synaptic vesicle docking cascade must be understood for it is the inhibition of the release of the neurotransmitter acetylcholine from cholinergic motor neurons which leads to the classic flaccid paralysis seen in botulinum intoxicated muscle tissue.
The key event in the release of neurotransmitter is exocytosis of the synaptic vesicle contents through fusion of the synaptic vesicles with the phospholipid plasma membrane. Normally, synaptic vesicles are predocked on the inside of the plasma membrane through a series of docking proteins and acetylcholine molecules are exocytotically released by an increase in the intracellular Ca.sup.+2 concentration (Sudhof, T., Nature 375:645-653, 1995). The docking proteins and their relationship to the synaptic vesicles is shown in FIG. 2.
The neurotoxic activity of all seven serotypes of neurotoxin is related to the fact that the light chains of botulinum toxin as well as the light chain of tetanus toxin are known to be zinc endopeptidases. The zinc binding region of the light chain of the neurotoxins is highly conserved and is very homologous between the serotypes. It includes a region that possesses the zinc binding motif HExxH surrounded by sequences that show a lesser degree of homology. The intracellular target for each serotype is one or more of the proteins involved in docking of the acetylcholine containing vesicles to the neuronal membrane. Cleavage of the various neurotoxin substrates inhibits the docking of the vesicles with the plasma membrane and, hence, the release of the neurotransmitter into the synaptic junction. The various substrates for the seven serotypes of botulinum neurotoxin as well as tetanus toxin are shown in Table 2.
TABLE 2 ______________________________________ Intracellular substrates of clostridial neurotoxins (adapted from Oguma, K. et al., Microbiol. Immunol. 39:161-168, 1995). Neurotoxin serotype Intracellular target Cleavage site ______________________________________ A SNAP-25 Gln197--Arg198 B Synaptobrevin-2 Glu76--Phe77 (VAMP-2) C.sub.1 Syntaxin near C-terminus D Synaptobrevin-1 Lys61--Leu62 (VAMP-1) Synaptobrevin-2 Lys59--Leu60 (VAMP-2) E SNAP-25 Arg180--Ile181 F Synaptobrevin-1 Gln60--Lys61 (VAMP-1) Synaptobrevin-2 Gln58--Lys59 (VAMP-2) G Synaptobrevin-1 Ala83--Ala84 (VAMP-1) Synaptobrevin-2 Ala81--Ala82 (VAMP-2) Tetanus toxin Synaptobrevin-2 Glu76--Phe77 ______________________________________
Because patients have developed immunity to treatment with type A botulinal toxin (Borodic, G., et al., Neurology 46:26-29, 1996), a toxin preparation that avoids that immunological problem is highly desired.